Pastel is not colored chalk, which is a limestone substance. Pastel is pure pigment-the same pigment used in making all fine art paints. It is the most permanent of all media when applied to a permanent ground and properly framed. There is no oil to cause darkening or cracking, nor other substance or medium to cause fading or blistering. Pastels from the 16th Century exist today, as fresh and alive as the day they were painted!
courtesy of the Connecticut Pastel Society
Pastel paintings require the surface be protected by glazing (glass being the most preferred). Pastel works of art generally require a minimum of 1/4? between the surface of the pastel and the glazing. The glazing should be cleaned on both sides with anti-static cleaner especially if Plexiglas or acrylic glazing is used. Loose particles may be drawn to the inside surface of the glazing otherwise.
It is wise to insert a spacer between the surface of the pastel and the mat for loose pastel particles to fall into. Before putting the pastel painting in place, tap the back to get rid of excess dust. Some artists spray their work but others prefer not to as the colours may change.
courtesy of IAPS
More about pastels from the Pastel Society of America.